Composite mixed carbonate ion and electron conducting membranes and reactant gas assisted chemical reactors for CO2 separation and capture

ABSTRACT

The present disclosure relates to a system for carbon dioxide separation and capture. The system includes a porous metal membrane comprising Ni, Ag, or combinations thereof and having molten carbonate within the pores. A CO2 containing flue gas input stream is separated from a reactant gas input stream by the membrane. The CO2 is removed from the flue gas input stream as it contacts the membrane resulting in a CO2 free flue gas output stream and a CO2 containing reactant gas output stream.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based on and claims priority to U.S. Provisional Application 61/335,753 having a filing date of Jan. 12, 2010, which is incorporated by reference herein.

BACKGROUND

Fossil fuels will be the primary energy resources to power society for at least the next fifty years. The use of fossil fuels generates CO₂. Unfortunately, CO₂ is a greenhouse gas, the emissions of which to the atmosphere cause global warming, an alarming threat to humankind. An increased awareness of this problem is recognized in the changes of energy policy at both national and international levels. For the scientific and engineering communities, research activities in searching for means of effectively reducing CO₂ emissions has also increased considerably in recent years. This trend will continue for a sustained period of time in near future.

The technical approach to reduce CO₂ emissions is simply to avoid the emissions to the atmosphere by capturing and storing it at either pre-combustion or post-combustion stage. At either stage, CO₂ has to be separated from either a reducing fuel stream (pre-combustion) or an oxidizing flue gas stream (post-combustion) into a highly concentrated form, from which CO₂ can then be further compressed into a liquid form and stored via geologic sequestration. The technologies available for the CO₂ separation include primarily mechanical scrubbing and poly-amine solvent-based physical adsorptions. The former is mainly used in separating CO₂ from CH₄ (or natural gas) and the latter is widely employed in CO₂ separation in flue and fuel gas streams of power plants. Unfortunately, both methods are energy intensive and cost prohibitive for large-scale commercial applications. For example, it is estimated that the cost will be at $50-$60 per metric ton of CO₂ captured, which translates to $100 million per year for a refinery to invest the CO₂ capture technology. Therefore, developing a cost-effective and separation-efficient CO₂ capture technology is both scientifically interesting and industrially demanding.

As such, a need exists for a CO₂ capture technology that addresses the shortcomings of conventional approaches. Methods of utilizing such technology would also be desirable.

SUMMARY

The present disclosure relates to a system for carbon dioxide separation and capture. The system includes a porous metal membrane comprising nickel (Ni), silver (Ag), or combinations thereof and having molten carbonate within the pores. A CO₂ containing flue gas input stream is separated from a reactant gas input stream by the membrane. The CO₂ is removed from the flue gas input stream as it contacts the membrane resulting in a CO₂ free flue gas output stream and a CO₂ containing reactant gas output stream.

In certain embodiments of the present disclosure, a method of carbon dioxide separation and capture is provided. The method includes contacting a CO₂ containing flue gas input stream with a porous metal membrane comprising nickel (Ni), silver (Ag), or combinations thereof and having molten carbonate within the pores. The membrane separates the CO₂ containing flue gas input stream from a reactant gas input stream. The CO₂ is removed from the flue gas input stream as it contacts the membrane resulting in a CO₂ free flue gas output stream and a CO₂ containing reactant gas output stream.

BRIEF DESCRIPTION OF THE FIGURES

A full and enabling disclosure of the present subject matter, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:

FIG. 1 illustrates a conceptual cross-sectional schematic of the mixed carbonate ion and electron conducting membrane in accordance with certain embodiments of the present disclosure.

FIG. 2 illustrates a schematic of the CO₂ separation reactor based on mixed conducting membrane in accordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each example is provided by way of explanation of the subject matter, not limitation of the subject matter. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment.

The present disclosure describes a novel mixed carbonate ion and electron conducting membrane that is capable of separating CO₂ from a flue gas stream. Different from the current poly-amine solvent-based physical-adsorption-in-principle technology, the proposed composite mixed conducting CO₂ separation membrane is based on electrochemical principles, and therefore has a sole selectivity to CO₂ but no limitation to the separation efficiency. The chemical reactors that are built on the membrane and assisted by the reactant gases are also presented.

The disclosed mixed carbonate ion and electron conductor is comprised of two phases. One phase is the porous substrate made of metal such as pure Ni, pure Ag, or combinations thereof. Another phase is the molten carbonates that are held within pores of the porous metal substrate by the capillary force. FIG. 1 shows the conceptual cross-sectional schematic of the composite mixed conductor in a tubular geometry. The functionality of the porous metal substrate is multiple. First, it provides the pathway for electrons to react at the gas-solid-liquid surfaces. Second, it also provides the physical pores for holding the molten electrolyte by the capillary force and mechanical structure for the membrane. During separation, the carbonate ions in molten carbonate electrolytes migrate from high end of chemical potentials of CO₂ and O₂ in the flue gas stream to the low end in the reactant gas. The electrode reactions at the two reactive surfaces are represented by: At CO₂-rich side with flue gas: CO₂(h)+½O₂(h)+2e ⁻=CO₃ ²⁻  (1) At CO₂-lean side with reactant gas A: A(l)+CO₃ ²⁻=AO(l)+CO₂(l)+2e ⁻  (2) Overall reaction: CO₂(h)+½O₂(h)+A(l)=AO(l)+CO₂(l)  (3)

Reaction (3) depicts the completion of the separation of CO₂ from one side (high end) to another side (low end).

Selection of the reactant gas A is critical to the design of chemical reactors and successful separation of CO₂. It is apparent that the formed mixture of AO and CO₂ in the effluent has to be easily separable to allow a true separation of CO₂. One candidate reactant gas is H₂, with which a mixture of H₂O and CO₂ can be formed in the effluent as a result of the reaction (2). A simple physical condensation of H₂O at the downstream of the reactor can then be applied to obtain a pure stream of CO₂. Another candidate is CO, with which only CO₂ is the product after reacting with the membrane. No secondary separation is needed in this case. In practice, the mixture of H₂ and CO is an easier and more common form to obtain by traditional industrial processes such as coal gasification or methane reforming. Therefore, in FIG. 2 both kinds of reactant gases are used to illustrate the principle of the CO₂ separation chemical reactor built upon the proposed mixed conducting membrane. As shown, a component labeled Coal Gasification Reactor or Steam Methane Reactor is added to regenerate CO and H₂ that are needed for the subsequent CO₂ separation reactions by allowing a fraction of produced H₂O and CO₂ to be recycled. The electrode and overall chemical reactions with H₂ and CO as the reactant gases are represented by At flue gas side: CO₂(h)+½O₂(h)+2e ⁻=CO₃ ²⁻  (4) At reactant gas side: H₂(l)+CO₃ ²⁻=H₂O(l)+CO₂(l)+2e ⁻  (5) CO(l)+CO₃ ²⁻=2CO₂(l)+2e ⁻  (6) Overall reaction: H₂(l)+CO₂(h)+½O₂(h)=H₂O(l)+CO₂(l)  (7) CO(l)+CO₂(h)+½O₂(h)=2CO₂(l)  (8)

At the end, the chemical reactor produces two product lines: one is the CO₂-free stream containing only N₂ and H₂O; another is the pure stream of CO₂ after physical condensation of H₂O.

In the interests of brevity and conciseness, any ranges of values set forth in this specification are to be construed as written description support for claims reciting any sub-ranges having endpoints which are whole number values within the specified range in question. By way of a hypothetical illustrative example, a disclosure in this specification of a range of 1-5 shall be considered to support claims to any of the following sub-ranges: 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.

These and other modifications and variations to the present disclosure can be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present disclosure, which is more particularly set forth in the appended claims. In addition, it should be understood that aspects of the various embodiments can be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the disclosure so further described in such appended claims. The metals Ni and Ag are used herein to illustrate the porous metal membrane of the present invention but are not intended to limit the scope of the invention. 

What is claimed:
 1. A system for carbon dioxide separation and capture comprising: a porous metal membrane comprising Ni, Ag, or combinations thereof and having molten carbonate within the pores; a CO₂ and O₂ containing flue gas input stream configured to contact a first surface of the membrane; and a reactant gas input stream containing reactants consisting of CO and/or H₂,the flue gas input stream being separated from the reactant gas input stream by the membrane, the reactant gas input stream being, configured to contact a second surface of the membrane, wherein the CO₂ and O₂of the flue gas input stream react as the flue gas input stream contacts the first surface of the membrane to form a carbonate ion according to the following reaction: CO₂+½O₂+2e ⁻→CO₃ ²⁻, the carbonate ion migrating across the porous membrane to contact the reactant gas input stream at the second surface of the membrane, the carbonate ion reacting with the CO and/or H₂ of the reactant gas input stream according to the following reactions: H₂+CO₃ ²⁻→H₂O+CO₂+2e ⁻ and/or CO +CO₃ ²⁻→2CO₂+2e ⁻ resulting in a CO₂ free flue gas output stream and a CO₂ containing reactant gas output stream.
 2. A system as in claim 1, wherein the CO₂ and O₂ containing flue gas input stream comprises N₂.
 3. A system as in claim 1, wherein the CO₂ and O₂ containing flue gas input stream comprises H₂O.
 4. A system as in claim 1, wherein the reactant of the, reactant gas input stream consists of CO.
 5. A system as in claim 1, wherein the reactant of the, reactant gas input stream consists of H₂.
 6. A system as in claim 1, wherein the CO₂ free flue gas output stream comprises N₂.
 7. A system as in claim 1, wherein the CO₂ free flue gas output stream comprises H₂O.
 8. A system as in claim 1, wherein the CO₂ containing reactant gas output stream comprises H₂O.
 9. A system as in claim 8, wherein H₂O is condensed from the CO₂ containing reactant gas output stream.
 10. A method of carbon dioxide separation and capture comprising: contacting a CO₂ and O₂ containing flue gas input stream with a first surface of a porous metal membrane comprising Ni, Ag, or combinations thereof and having molten carbonate within the pores, the membrane separating the CO₂ and O₂ containing flue gas input stream from a reactant gas input stream containing reactants consisting of CO and/or H₂, the reactant gas input stream contacting a second surface of the membrane, wherein the CO₂ and O₂ of the flue gas input stream react as the flue gas input stream contacts the first surface of the membrane to form a carbonate ion according to the following reaction: CO₂+½O₂+2e ⁻→CO₃ ²⁻, the carbonate ion migrating across the porous membrane to contact the reactant gas input stream at the second surface of the membrane, the carbonate ion reacting with the CO and/or H₂ according to the following reactions: H₂+CO₃ ²⁻→H₂O+CO₂+2e ⁻and/or CO+CO₃ ²⁻→2CO₂+2e ⁻ resulting in a CO₂ free flue gas output stream and a CO₂ containing reactant gas output stream.
 11. A method as in claim 10, wherein the CO₂ and O₂containing flue gas input stream comprises N₂.
 12. A method as in claim 10, wherein the CO₂ and O₂containing flue gas input stream comprises H₂O.
 13. A method as in claim 10, wherein the reactant of the reactant gas input stream consists of comprises CO.
 14. A method as in claim 10, wherein the reactant of the reactant gas input stream consists of H₂.
 15. A method as in claim 10, wherein the CO²free flue gas output stream comprises N₂.
 16. A method as in claim 10, wherein the CO₂ free flue gas output stream comprises H₂O.
 17. A method as in claim 10, wherein the CO₂ containing reactant gas output stream comprises H₂O.
 18. A method as in claim 17, wherein H₂O is condensed from the CO₂ containing reactant gas output stream.
 19. The system as in claim 1, wherein the reactants of the reactant gas input stream consist of CO and H₂.
 20. The system as in claim 19, further comprising a coal gasification reactor or a steam methane reactor, wherein the reactant gas stream is generated in the coal gasification reactor or the steam methane reactor.
 21. The method as in claim 10, wherein the reactants of the reactant gas input stream consist of CO and H₂.
 22. The method as in claim 21, further comprising generating the reactant gas input stream in a coal gasification reactor or a steam methane reactor. 